1. Field of the Invention
This invention relates to a magnetic metal powder suitable for high-density magnetic recording and, more particularly, to a magnetic metal powder with high coercive force, in which the metal contains a certain amount of carbon in the form of metal carbide.
2. Background Art
With recent demands for high-density magnetic recording, practical use has been made of magnetic powders having high coercive force and residual magnetization and formed of iron or an alloy composed mainly of iron in place of iron oxides heretofore used for recording medium purposes such as .gamma.-Fe.sub.2 O.sub.3 with or without Co-coating thereon.
Most of currently available magnetic metal powders are obtained by the oxidization of the starting needle-like goethite or hematite, followed by reduction in a hydrogen stream. Such metal powders have improved coercive force due to the specific shape anisotropy of their particles, i.e. the needle-like shape of starting iron oxyhydroxides which is retained even after reduction. With these metal powders, however, it is quite difficult to control the particle size at a level of 300 angatrom (.ANG.) or lower, which is a level requisite for the electro-magnetic conversion characteristics of a recording medium, since the mechanism of particle-forming during reduction is so complicated and sintering tends to take place among particles in the course of reduction.
Moreover, the resulting fine metal particles tend to have a rough surface structure and are liable to be very active due to an extended period of reduction, the dehydration reaction of the starting iron oxyhydroxide, etc., whereby they are subject to oxidation in the air. This offers a drawback of poor weathering resistance even after they have been dispersed in a binder to form a coating film.
Thus, there has been a demand for magnetic metal powders substantially free from the above mentioned problems.
As one of the methods for producing such magnetic metal powders, it has been known to subject transition metal carbonyl compounds to gas-phase thermal decomposition as disclosed in, for instance, Japanese Patent Publication Nos. 24316/1968, 11529/1969 and 31809/1977 as well as U.S. Pat. Nos. 2,983,997 and 2,884,319. According to such proposals, the metal carbonyl compounds are decomposed to directly form metal particles which have smoother and more stable surfaces as compared with those obtained by the aforesaid hydrogen reduction methods. These metal particles may thus be suited for coatings. However, the metal particles obtained by this known technique fail to possess such a high coercive force as required for high-density recording, partly because they have too large a particle diameter and contain too large an amount of carbon and partly because the carbon contained therein is not converted into a metal carbide.